Magnetostriction type torque sensor shafts have been known, as one is shown in Japanese Patent No. 169,326, wherein the surface of a sensor shaft adapted to have torque transmitted thereto is formed with a magnetically anisotropic section by forming spiral grooves therein by cutting or rolling so as to detect changes in the permeability of the magnetic anisotropic section, when torque is applied, to expess them in terms of electrical quantities.
Hitherto, however, no torque sensor of such grooved type has been put in actual use in the art. The reason for this is that a torque sensor shaft constructed of a structural steel material through a mere process such that the material is formed with spiral grooves and then subjected to suitable heat treatment is liable to hysteresis, usually of the order of about 2 to 20% FS, and could not be used as such in any practical application. Recently, in order to put the basic principle of such torque sensor shaft into practical use, a knurled type magnetostrictive torque sensor has been proposed as disclosed in U.S. Pat. No. 4,933,580 of the present inventors, wherein shot peening is applied to the grooved portion to decrease hysteresis and improve sensitivity.
The achievement of hysteresis reduction through the process of peening the grooved portion after heat treatment as described in U.S. Pat. No. 4,933,580 is explained by the fact that broadly peening has two kinds of effect, mechanical and magnetic.
More specifically, the mechanical effect of shot peening includes the effect of mending microcracks produced during the process of groove forming, and the effect of improving the mechanical strength. The improvement of the mechanical strength is brought about in the form of hardened surface layer of the shaft and reduced crystal grain size of the surface layer which result from the collision of small shot particles blown. Such mechanical effect results in reduced hysteresis of the sensor.
The magnetic effect of shot peening includes improvement in the process of surface magnetization and intensification of the magnetic anisotropy of the sensor. The improvement of surface magnetization occurs as a result of the fact that by virtue of shot peening the process of surface magnetization changes from magnetization through domain wall displacement, a process which tends to cause magnetic hysteresis, to revolving magnetization, a process which is unlikely to cause magnetic hysteresis. Such improvement in the process of magnetization results in reduced hysteresis and improved sensor sensitivity. The intensification of the magnetic anisotropy results from the fact that shot peening induces development of residual stress on the shaft material as will be described hereinafter. By virtue of the intensified magnetic anisotropy the non-linearity of the sensor is corrected.
In the past, the effect of shot peening was well known in that the hardness of the outermost surface layer of the shaft was increased and some compressive residual stress was provided, which would result in improved mechanical strength (fatigue strength). However, the past recognition in the art that the effect of shot peening was limited to the improvement of such mechanical strength involves the following inconsistency. While, as is well known, shot peening brings about increased hardness of the surface layer of the material and, in conjunction therewith, improved surface layer strength of the material, it must be pointed out that the magnetic hardness of the material is also increased and accordingly the retentivity of the material becomes so large that the material can hardly be magnetized further. As a matter of practice, therefore, mere improvement in mechanical strength is generally likely to lead to decreased sensitivity.
According to experiments conducted by the present inventors, wherein a shaft material was hardened by carburizing, tempered and heat treated to thereby increase the hardness of its surface layer, while for the purpose of comparison a similar shaft material was hardened in a carburization-prevented condition, and then tempered and heat treated, that is, bright-handened, tempered and heat treated without so much increase in the hardness of the shaft surface, the higher the shaft hardness resulting from carburization, the lower was the sensitivity of the torque sensor using the shaft.
As is apparent from this, the effect of shot peening presents some aspect that cannot be explained only on the basis of increased mechanical hardness and/or increased mechanical strength; and improvements in all sensor characteristics, such as reduced hysteresis, reduced non-liniarity, and improved sensitivity, can be obtained as an overall effect of shot peening, or a combination of mechanical effect and magnetic effect as above stated. In the earlier known shot peening technique as disclosed in, for example, U.S. Pat. No. 3,073,022, the effect of shot peening for mechanical strength improvement, as intended mainly for improvement of fatigue strength, was only taught. In the prior art, the above cited U.S. Pat. No. 4,933,580 was the first disclosure which referred to the above stated magnetic effect.
More particularly, in the invention of a magnetostrictive torque sensor described in U.S. Pat. No. 4,933,580, shot peening is applied to the grooved portion of a sensor shaft and to areas therearound thereby to mechanically, metallurgically and magnetically improve the outermost surface layer of the grooved portion to reduce hysteresis and increase sensitivity. That is, the following techniques are disclosed therein.
(1) Improvement of Mechanical and Metallurgical Strength of Grooves and Areas therearound:
Application of shot peening to grooves and areas therearound martensitizes the residual austenite in the outermost surface layer produced during carburization to shafts to thereby increase hardness and it decreases crystal grain size, whereby the strength of the outermost surface layer through which magnetic flux passes is increased to a great extent.
As a result, when torque is applied, sufficient strength is provided to resist the stress concentrated in the grooves. Even if a large stress is applied, there is little possibility of producing a macroscopic mechanical plastic deformation or a plastic deformation on the microscopic crystalline level which causes the first mentioned plastic deformation or, in other words, a magnetic plastic deformation. As a result, the hysteresis characteristic is improved.
An auxiliary effect obtained is that the sensitivity is increased owing to the nonmagnetic residual austenite being converted to ferromagnetic martensite.
(2) Effect of Mending Microcracks in Grooves and Areas therearound:
Generally, microcracks are often produced in grooves and areas therearound during machining, particularly rolling.
Such microcracks aggravate the hysteresis characteristic and lower the sensitivity of sensors.
As is well known, shot peening has the effect of mending such microcracks and hence it is useful for improving the hysteresis characteristic and increasing the sensor sensitivity.
(3) Improvements in Magnetization of Outermost Surface Layers of Sensors:
Usually, magnetostrictive sensors are magnetized in a low magnetic field having a magnetic intensity of tens of oersteds at 10 kHz to 100 kHz. In most cases, the skin depth is about 0.1 mm immediately below the outermost surface layer and the magnetization process is based mostly on the domain wall displacement.
In this case, the presence of impurities and nonmagnetic inclusions in the skin depth region forms a cause of magnetic hysteresis, and since shaft materials in common use cannot avoid these impurities, it has been usual that the hysteresis characteristic is bad.
On the other hand, application of shot peening results in forming microscopic unevenness in the outermost surface layer of the grooves and areas therearound which form the magnetically anisotropic section of the sensor.
According to the teachings of physics of magnetism, formation of microscopic pits on a metal surface by plastic deformation results in formation of stable magnetic domains around the microscopic pits due to annular residual stress, with the magnetization process in the annular stable magnetic domains converting to magnetization rotation with less magnetic hysteresis, thereby lowering the hysteresis characteristic of the sensor.
With the above effects organically coupled together, application of shot peening brings about a decrease in hysteresis and an increase in sensitivity.
The action based on the effect (3) above is based on the action of residual stress distribution applied to a region in the vicinity of the outermost surface layer.
According to the known technique as described in the above cited U.S. Pat. No. 4,933,580, when shot peening is applied to the surface of a sensor shaft, only shot particles of uniform size are used. If the size of shot particles is large, a stress distribution in which the compressive residual stress is at a maximum and approximately constant is formed in a deep region below the shaft surface in a wide range as seen in the direction of the depth. Further, if the size of shot particles is small, a peak value of compressive residual stress is obtained in a shallow region, but in this case the region where the compressive residual stress is approximately constant is narrow.
For this reason, in the case where the size of shot particles is small, excitation conditions using high frequency ac currents are utilized to ensure shallow penetration of magnetic flux; in this manner, optimum excitation conditions are provided. However, since the region where the compressive residual stress is approximately constant is narrow, it is necessary that the range of utilizable excitation frequencies be from 50 kHz to 100 kHz, which are considerably high frquencies.
In the case where the size of shot particles is large, excitation conditions using low frequencies (usually, about 10 kHz) to enable magnetic flux to penetrate into depths are selected so that the depth of penetration of magnetic flux is greater than when the size of shot particles is small and so as to minimize the influence on the outermost surface layer of the shaft where the changes in stress distribution is large and where the compressive residual stress is small; in this manner, hysteresis and sensitivity are improved. However, in this case, the region of the shaft near its surface aggravates the sensor characteristics. Therefore, when it is desired to obtain satisfactory hysteresis characteristics, it is necessary to use a large excitation current.
In torque sensor shafts having such conventional shot peening methods applied thereto, the range in which the compressive residual stress is approximately constant does not necessarily have a sufficient expanse, so that there is a problem that the range of usable excitation frequencies is narrow.